feat(openai): 同步生图 API 支持并接入图片计费调度

- 同步 OpenAI 图片生成与编辑接口
- 接入图片请求解析、账号调度、转发与用量记录
- 接入图片计费与图片用量落库
- 限制 OAuth 生图仅支持无显式模型和尺寸的基础请求

backend/internal/server/middleware/security_headers.go

@@ -96,7 +96,8 @@ func isAPIRoutePath(c *gin.Context) bool {
 	return strings.HasPrefix(path, "/v1/") ||
 		strings.HasPrefix(path, "/v1beta/") ||
 		strings.HasPrefix(path, "/antigravity/") ||
-		strings.HasPrefix(path, "/responses")
+		strings.HasPrefix(path, "/responses") ||
+		strings.HasPrefix(path, "/images")
 }
 
 // enhanceCSPPolicy ensures the CSP policy includes nonce support, Cloudflare Insights,